The present invention relates to a variable displacement pump which serves as a source for supplying the hydraulic pressure to a hydraulic device such as an automotive power steering apparatus.
Typically, the variable displacement pump comprises a housing, a cam ring arranged swingably in the housing, first and second fluid-pressure chambers arranged in one and another swing directions of the cam ring, respectively, and a spring arranged in the second fluid-pressure chamber and for biasing the cam ring to the first fluid-pressure chamber.
Arranged rotatably in the cam ring is a vane rotor having an axis offset with respect to the center of the cam ring. The vane rotor comprises slots formed radially in the outer periphery and vanes held therein to be movable with respect to the inner-peripheral surface of the cam ring.
A metering orifice is provided to a discharge passage for supplying to a hydraulic device pressurized fluid discharged from a pump chamber formed between the cam ring and each vane to a discharge port. A control valve comprises a spool arranged slidably in a valve hole by the pressure difference between the upstream and downstream sides of the metering orifice. The fluid pressure within the first fluid-pressure chamber is controlled in accordance with the slide position of the spool.
Specifically, the control valve comprises a high-pressure chamber formed at one end of the spool and for introducing pressurized fluid on the upstream side of the metering orifice, a pressure chamber formed at another end of the spool and for introducing pressurized fluid on the suction side of the pump, and a low-pressure chamber comprising an annular groove formed substantially in the center of the outer-peripheral surface of the spool and for introducing pressurized fluid in a tank. The control valve controls internal pressure such that, in accordance with slide motion of the spool to the pressure chamber, a cylindrical valve element formed on the outer periphery of the spool carries out selective switching from the low-pressure chamber to the high-pressure chamber so as to supply fluid in the high-pressure chamber to the first fluid-pressure chamber.
The second fluid-pressure chamber is isolated from the control valve to always introduce therein pressure from the suction side of the pump.
Therefore, during low rotation of the pump, the control valve is not operated due to small pressure difference between the upstream and downstream sides of the metering orifice, so that the first fluid-pressure chamber is supplied with low-pressure (atmospheric-pressure) fluid in the tank. Thus, the cam ring is biased to the first fluid-pressure chamber by a biasing force of the spring arranged in the second fluid-pressure chamber, having center offset with respect to the center of the vane rotor. This increases volume of the pump chambers formed between the vanes and the cam ring and on the side of the second fluid-pressure chamber, allowing the power steering apparatus to be supplied with sufficient flow rate of pressurized fluid through the discharge port and the discharge passage.
As the pump is in high rotation, the spool of the control valve is operated to the pressure chamber due to great pressure difference between the upstream and downstream sides of the metering orifice, so that the valve element of the control valve is moved to carry out switching from the low-pressure (tank-pressure) chamber to the high-pressure chamber. Thus, highly pressurized fluid discharged to the discharge port is supplied to the first fluid-pressure chamber so as to swing the cam ring to the second fluid-pressure chamber against a biasing force of the spring, controlling the volume of the pump chambers at a small value. With this, the power steering apparatus is supplied with a predetermined flow rate of pressurized fluid, ensuring constant flow rate thereof.
During low rotation of the pump where it is desirable to ensure the pump discharge, only the fluid pressure within the low-pressure chamber operates on the second fluid-pressure chamber as described above, so that leakage of pressurized fluid from the second fluid-pressure chamber to the outside is prevented from occurring, allowing the pump discharge to be ensured sufficiently.